Reddit:

• It has a much larger user base and many heavily specialized boards that nevertheless stay reasonably active.
• It’s a collection of echo chambers. Dissent is usually stomped out by mass downvoting and heavy moderation/bans. It’s rare to find a board that allows arguments for a long period of time. Agree with the board’s users/mods or get silenced. Posted rules do not matter, and you can definitely be hateful in ways that violate posted rules so long as that type of hate is acceptable on that board.
• So many users mean that getting content to succeed is a crapshoot. Often posts become lost in the noise, especially on busy boards.
• I left about a year ago, but apparently there’s a lot of bot/AI slop on boards now.

Lemmy:

• Much smaller user base. Heavily specialized boards move slowly if they exist at all. It’s not unusual to see boards where it’s just one/a few people posting with days in between new content.
• More ability to have disagreements. Whether it’s because moderating a smaller # of users is easier, the mods are less authoritarian, or whatever you are more likely to be able to disagree. Don’t be blatantly racist, celebrating violence, clearly trolling, etc. and you’ll probably remain able to participate. I’m sure this isn’t universal on all boards, but it’s my experience on many boards.
• For all that I believe the above point, there are still “echo chamber” moments on Lemmy. Sometimes it seems people may be downvoted simply because they are already downvoted. It’s still way less egregious than on Reddit, and such is human nature I suppose.
• Fewer users means you are more likely to get some engagement on your post, at least in my experience. I never sorted my feed by new posts on Reddit because it was an avalanche of posts of questionable quality, so I only saw whatever content had already succeeded. On Lemmy I can look for new posts and see most if not all content on the boards I enjoy.